[show abstract][hide abstract] ABSTRACT: The positron range, that is, the distance from the positron emission point to the annihilation one in β+ decay, limits the spatial resolution of PET images. Thus, any realistic simulation of PET acquisitions must include this effect as accurately as possible. Positron range distributions have been computed and compared in the literature. Not all authors present it in the same way. For instance, accumulated range distributions, 1D projection of the positrons annihilation coordinates, or angular integrated 3D radial distributions, have been used. Any of these presentations may emphasize a different physical property. Thus it is difficult to perform a meaningful comparison of the results obtained by different authors. The aim of this work is to present a general framework to compare positron range distributions. We use a genetic algorithm to fit the radial, angle integrated, g3D(r) distribution of the annihilation points to any positron annihilation distribution chosen as reference. Therefore, given a positron range distribution presented in a certain way, we can obtain the distributions, first fitting g3D and then using the relations between distributions. Using this procedure, positron range distributions available in the literature are compared with the ones computed with the simulation package PeneloPET
[show abstract][hide abstract] ABSTRACT: Technical advances towards high resolution PET imaging try to overcome the inherent physical limitations to spatial resolution. Positrons travel in tissue until they annihilate into the two gamma photons detected. This range is the main detector-independent contribution to PET imaging blurring. To a large extent, it can be remedied during image reconstruction if accurate estimates of positron range are available. However, the existing estimates differ, and the comparison with the scarce experimental data available is not conclusive. In this work we present positron annihilation distributions obtained from Monte Carlo simulations with the PeneloPET simulation toolkit, for several common PET isotopes ((18)F, (11)C, (13)N, (15)O, (68)Ga and (82)Rb) in different biological media (cortical bone, soft bone, skin, muscle striated, brain, water, adipose tissue and lung). We compare PeneloPET simulations against experimental data and other simulation results available in the literature. To this end the different positron range representations employed in the literature are related to each other by means of a new parameterization for positron range profiles. Our results are generally consistent with experiments and with most simulations previously reported with differences of less than 20% in the mean and maximum range values. From these results, we conclude that better experimental measurements are needed, especially to disentangle the effect of positronium formation in positron range. Finally, with the aid of PeneloPET, we confirm that scaling approaches can be used to obtain universal, material and isotope independent, positron range profiles, which would considerably simplify range correction.
Physics in Medicine and Biology 07/2013; 58(15):5127-5152. · 2.70 Impact Factor
[show abstract][hide abstract] ABSTRACT: In this work we use PeneloPET to simulate the Biograph scanners B-TP,
B-TPTV and mCT. Simulations were adjusted to reproduce some experimental
results from the actual scanners and validated by comparing their
predictions to further experimental results. Sensitivity, spatial
resolution, NEC rate and scatter fraction (SF) were estimated.
[show abstract][hide abstract] ABSTRACT: Positron range limits the spatial resolution of PET images and has a
different effect for different isotopes and positron propagation
materials. In this work we present a positron range distributions for
different isotopes and materials computed with PeneloPET. We study also
the possibility of scaling these range profiles in order to obtain
universal profiles independent of the density of materials and energy of
the emitter. To obtain material independent profiles, a scaling by the
density of material was performed, while to compute a universal curve
for different emitters, we scaled by the mean range of the positrons for
the isotope considered. Scaling in the material was very good and
scaling with the positron energy was good enough. This range profiles
obtained from the universal profiles and the scaling prescription differ
by less than 5% in terms of mean positron range for the different
materials and isotopes considered in this work.
[show abstract][hide abstract] ABSTRACT: Pile-up and dead-time are two main causes of nonlinearity in the response of a PET scanner as a function of activity in the field of view (FOV). For a given scanner and acquisition system, pile-up effects depend on the material and size of the object being imaged and on the distribution of activity inside and outside the FOV, because these factors change the singles-to-coincidences ratio (SCR). Thus, it is difficult to devise an accurate correction that would be valid for any acquisition. In this work, we demonstrate a linear relationship between SCR and effective dead-time, which measures the effects of both dead-time (losses) and pile-up (gains and losses). This relationship allows us to propose a simple method to accurately estimate dead-time and pile-up corrections using only two calibration acquisitions with, respectively, a high and low SCR. The method has been tested with simulations and experimental data for two different scanner geometries: a scanner with large area detectors and no pile-up rejection, and a scanner composed of two full rings of smaller detectors. Our results show that the SCR correction method is accurate within 7%, even for high activities in the FOV, and avoids the bias of the standard single-parameter method.
Physics in Medicine and Biology 03/2013; 58(7):2059-2072. · 2.70 Impact Factor
[show abstract][hide abstract] ABSTRACT: Technological advances have improved the assembly process of PET detectors, resulting in quite small mechanical tolerances. However, in high-spatial-resolution systems, even submillimetric misalignments of the detectors may lead to a notable degradation of image resolution and artifacts. Therefore, the exact characterization of misalignments is critical for optimum reconstruction quality in such systems. This subject has been widely studied for CT and SPECT scanners based on cone beam geometry, but this is not the case for PET tomographs based on rotating planar detectors. The purpose of this work is to analyze misalignment effects in these systems and to propose a robust and easy-to-implement protocol for geometric characterization. The result of the proposed calibration method, which requires no more than a simple calibration phantom, can then be used to generate a correct 3D-sinogram from the acquired list mode data.
Physics in Medicine and Biology 10/2012; 57(22):7493-7518. · 2.70 Impact Factor
[show abstract][hide abstract] ABSTRACT: This work presents a graphics processing unit (GPU)-based implementation of a fully 3-D PET iterative reconstruction code, FIRST (Fast Iterative Reconstruction Software for [PET] Tomography), which was developed by our group. We describe the main steps followed to convert the FIRST code (which can run on several CPUs using the message passing interface [MPI] protocol) into a code where the main time-consuming parts of the reconstruction process (forward and backward projection) are massively parallelized on a GPU. Our objective was to obtain significant acceleration of the reconstruction without compromising the image quality or the flexibility of the CPU implementation. Therefore, we implemented a GPU version using an abstraction layer for the GPU, namely, CUDA C. The code reconstructs images from sinogram data, and with the same System Response Matrix obtained from Monte Carlo simulations than the CPU version. The use of memory was optimized to ensure good performance in the GPU. The code was adapted for the VrPET small-animal PET scanner. The CUDA version is more than 70 times faster than the original code running in a single core of a high-end CPU, with no loss of accuracy.
IEEE Transactions on Nuclear Science 11/2011; · 1.22 Impact Factor
[show abstract][hide abstract] ABSTRACT: In vivo PET range verification relies on the comparison of measured and simulated activity distributions. The accuracy of the simulated distribution depends on the accuracy of the Monte Carlo code, which is in turn dependent on the accuracy of the available cross-section data for β(+) isotope production. We have explored different cross-section data available in the literature for the main reaction channels ((16)O(p,pn)(15)O, (12)C(p,pn)(11)C and (16)O(p,3p3n)(11)C) contributing to the production of β(+) isotopes by proton beams in patients. Available experimental and theoretical values were implemented in the simulation and compared with measured PET images obtained with a high-resolution PET scanner. Each reaction channel was studied independently. A phantom with three different materials was built, two of them with high carbon or oxygen concentration and a third one with average soft tissue composition. Monoenergetic and SOBP field irradiations of the phantom were accomplished and measured PET images were compared with simulation results. Different cross-section values for the tissue-equivalent material lead to range differences below 1 mm when a 5 min scan time was employed and close to 5 mm differences for a 30 min scan time with 15 min delay between irradiation and scan (a typical off-line protocol). The results presented here emphasize the need of more accurate measurement of the cross-section values of the reaction channels contributing to the production of PET isotopes by proton beams before this in vivo range verification method can achieve mm accuracy.
Physics in Medicine and Biology 04/2011; 56(9):2687-98. · 2.70 Impact Factor
[show abstract][hide abstract] ABSTRACT: The purpose of this work was to compare the clinical adaptation of prompt gamma (PG) imaging and positron emission tomography (PET) as independent tools for non-invasive proton beam range verification and treatment validation. The PG range correlation and its differences with PET have been modeled for the first time in a highly heterogeneous tissue environment, using different field sizes and configurations. Four patients with different tumor locations (head and neck, prostate, spine and abdomen) were chosen to compare the site-specific behaviors of the PG and PET images, using both passive scattered and pencil beam fields. Accurate reconstruction of dose, PG and PET distributions was achieved by using the planning computed tomography (CT) image in a validated GEANT4-based Monte Carlo code capable of modeling the treatment nozzle and patient anatomy in detail. The physical and biological washout phenomenon and decay half-lives for PET activity for the most abundant isotopes such as (11)C, (15)O, (13)N, (30)P and (38)K were taken into account in the data analysis. The attenuation of the gamma signal after traversing the patient geometry and respective detection efficiencies were estimated for both methods to ensure proper comparison. The projected dose, PG and PET profiles along many lines in the beam direction were analyzed to investigate the correlation consistency across the beam width. For all subjects, the PG method showed on average approximately 10 times higher gamma production rates than the PET method before, and 60 to 80 times higher production after including the washout correction and acquisition time delay. This rate strongly depended on tissue density and elemental composition. For broad passive scattered fields, it was demonstrated that large differences exist between PG and PET signal falloff positions and the correlation with the dose distribution for different lines in the beam direction. These variations also depended on the treatment site and the particular subject. Thus, similar to PET, direct range verification with PG in passive scattering is not easily viable. However, upon development of an optimized 3D PG detector, indirect range verification by comparing measured and simulated PG distributions (currently being explored for the PET method) would be more beneficial because it can avoid the inherent biological challenges of the PET imaging. The improved correlation of PG and PET with dose when using pencil beams was evident. PG imaging was found to be potentially advantageous especially for small tumors in the presence of high tissue heterogeneities. Including the effects of detector acceptance and efficiency may hold PET superior in terms of the amplitude of the detected signal (depending on the future development of PG detection technology), but the ability to perform online measurements and avoid signal disintegration (due to washout) with PG are important factors that can outweigh the benefits of higher detection sensitivity.
Physics in Medicine and Biology 02/2011; 56(4):1063-82. · 2.70 Impact Factor
[show abstract][hide abstract] ABSTRACT: Positron range limits the spatial resolution of PET images and has a different effect for different isotopes and positron propagation materials. Therefore it is important to consider it during image reconstruction, in order to obtain optimal image quality. Positron range distributions for most common isotopes used in PET in different materials were computed using the Monte Carlo simulations with PeneloPET. The range profiles were introduced into the 3D OSEM image reconstruction software FIRST and employed to blur the image either in the forward projection or in the forward and backward projection. The blurring introduced takes into account the different materials in which the positron propagates. Information on these materials may be obtained, for instance, from a segmentation of a CT image. The results of introducing positron blurring in both forward and backward projection operations was compared to using it only during forward projection. Further, the effect of different shapes of positron range profile in the quality of the reconstructed images with positron range correction was studied. For high positron energy isotopes, the reconstructed images show significant improvement in spatial resolution when positron range is taken into account during reconstruction, compared to reconstructions without positron range modeling.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2011; 648. · 1.14 Impact Factor
[show abstract][hide abstract] ABSTRACT: Fully 3D iterative tomographic image reconstruction is computationally very demanding. Graphics Processing Unit (GPU) has been proposed for many years as potential accelerators in complex scientific problems, but it has not been used until the recent advances in the programmability of GPUs that the best available reconstruction codes have started to be implemented to be run on GPUs.This work presents a GPU-based fully 3D PET iterative reconstruction software. This new code may reconstruct sinogram data from several commercially available PET scanners. The most important and time-consuming parts of the code, the forward and backward projection operations, are based on an accurate model of the scanner obtained with the Monte Carlo code PeneloPET and they have been massively parallelized on the GPU. For the PET scanners considered, the GPU-based code is more than 70 times faster than a similar code running on a single core of a fast CPU, obtaining in both cases the same images. The code has been designed to be easily adapted to reconstruct sinograms from any other PET scanner, including scanner prototypes.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2011; 648. · 1.14 Impact Factor
[show abstract][hide abstract] ABSTRACT: Super Bialkali (SBA) photocathode is a new technology that improves the spectral response characteristics of position sensitive PMTs, boosting their quantum efficiency up to 35%. In this experiment, two SBA tubes were introduced into a production line of PET detectors mixed with the regular tubes. The detectors were assembled using the standard factory protocols for detector mounting, calibration and testing. We report an evaluation of the improvement introduced by the SBA photocathode comparing the spatial and energy resolution and the depth-of-interaction (DOI) performance of PET detector modules with DOI capabilities. We conclude that the superior performance of the SBA tube may enable the use of arrays with a larger number of crystals of smaller footprint, thus potentially improving the detector intrinsic spatial resolution without degrading the energy resolution or the phoswich (DOI) discrimination capability.
IEEE Transactions on Nuclear Science 11/2010; · 1.22 Impact Factor
[show abstract][hide abstract] ABSTRACT: The Multi Pixel Photon Counter (MPPC) or Silicon Photomultiplier (SiPM) consists of an array of Geiger-mode photodiodes (microcells). It is a promising device for PET thanks to its high photon detection efficiency (PDE) and immunity to high magnetic fields. It is also very easy to use, with simple electronic read-out, high gain and small size. In this work we evaluate the performance of three 1×1 mm2 SiPM and one 2×2 SiPMs array (6×6 mm2 of active area). We examine the dependence of the energy resolution and the gain of these devices on the temperature and reverse bias when coupled to MLS and LYSO scintillator crystals. The good performance of these devices up to 7 Tesla has been also confirmed.
[show abstract][hide abstract] ABSTRACT: The multi-pixel photon counter (MPPC) or silicon photomultiplier (SiPM), recently introduced as a solid-state photodetector, consists of an array of Geiger-mode photodiodes (microcells). It is a promising device for PET due to its potential for high photon detection efficiency (PDE) and its foreseeable immunity to magnetic fields. It is also easy to use with simple read-outs, has a high gain and a small size. In this work we evaluate the in field performance of three 1×1mm2 (with 100, 400 and 1600 microcells, respectively) and one 6×6mm2 (arranged as a 2×2 array) Hamamatsu MPPCs for their use in PET imaging. We examine the dependence of the energy resolution and the gain of these devices on the temperature and reverse bias voltage, when coupled to LYSO scintillator crystals under conditions that one would find in a PET system. We find that the 400 and 1600 microcells models and the 2×2 array are suitable for small-size crystals, like those employed in high resolution small animal scanners. We have confirmed the good performance of these devices up to magnetic fields of 7T as well as their suitability for performing PET acquisitions in the presence of fast switching gradients and high duty radiofrequency MRI sequences.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2010; 613(2):308-316. · 1.14 Impact Factor
[show abstract][hide abstract] ABSTRACT: This work reports on the development and performance evaluation of the VrPET/CT, a new multimodality scanner with coplanar geometry for in vivo rodent imaging. The scanner design is based on a partial-ring PET system and a small-animal CT assembled on a rotatory gantry without axial displacement between the geometric centers of both fields of view (FOV). We report on the PET system performance based on the NEMA NU-4 protocol; the performance characteristics of the CT component are not included herein. The accuracy of inter-modality alignment and the imaging capability of the whole system are also evaluated on phantom and animal studies. Tangential spatial resolution of PET images ranged between 1.56 mm at the center of the FOV and 2.46 at a radial offset of 3.5 cm. The radial resolution varies from 1.48 mm to 1.88 mm, and the axial resolution from 2.34 mm to 3.38 mm for the same positions. The energy resolution was 16.5% on average for the entire system. The absolute coincidence sensitivity is 2.2% for a 100-700 keV energy window with a 3.8 ns coincident window. The scatter fraction values for the same settings were 11.45% for a mouse-sized phantom and 23.26% for a rat-sized phantom. The peak noise equivalent count rates were also evaluated for those phantoms obtaining 70.8 kcps at 0.66 MBq/cc and 31.5 kcps at 0.11 MBq/cc, respectively. The accuracy of inter-modality alignment is below half the PET resolution, and the image quality of biological specimens agrees with measured performance parameters. The assessment presented in this study shows that the VrPET/CT system is a good performance small-animal imager, while the cost derived from a partial ring detection system is substantially reduced as compared with a full-ring PET tomograph.
Physics in Medicine and Biology 09/2009; 54(18):5427-41. · 2.70 Impact Factor
[show abstract][hide abstract] ABSTRACT: Monte Carlo simulations play an important role in positron emission tomography (PET) imaging, as an essential tool for the research and development of new scanners and for advanced image reconstruction. PeneloPET, a PET-dedicated Monte Carlo tool, is presented and validated in this work. PeneloPET is based on PENELOPE, a Monte Carlo code for the simulation of the transport in matter of electrons, positrons and photons, with energies from a few hundred eV to 1 GeV. PENELOPE is robust, fast and very accurate, but it may be unfriendly to people not acquainted with the FORTRAN programming language. PeneloPET is an easy-to-use application which allows comprehensive simulations of PET systems within PENELOPE. Complex and realistic simulations can be set by modifying a few simple input text files. Different levels of output data are available for analysis, from sinogram and lines-of-response (LORs) histogramming to fully detailed list mode. These data can be further exploited with the preferred programming language, including ROOT. PeneloPET simulates PET systems based on crystal array blocks coupled to photodetectors and allows the user to define radioactive sources, detectors, shielding and other parts of the scanner. The acquisition chain is simulated in high level detail; for instance, the electronic processing can include pile-up rejection mechanisms and time stamping of events, if desired. This paper describes PeneloPET and shows the results of extensive validations and comparisons of simulations against real measurements from commercial acquisition systems. PeneloPET is being extensively employed to improve the image quality of commercial PET systems and for the development of new ones.
Physics in Medicine and Biology 03/2009; 54(6):1723-42. · 2.70 Impact Factor